Surface appearances in outside buildings exposed always to exterior environment are easily stained due to pollution factors such as dust or mote and rain or snow, so that routine management is required for cleaning. Therefore, to reduce increase of management expenses maximally, related industries have performed continuous researches.
In case of steel sheets for home electric appliances used almost indoors contrary to building materials exposed to outside, they were basically interrupted with pollution factors and not required for cleaning articles to be devoid of related researches. However, in case of air conditioners placed outside, they are exposed to pollution environment as such, so that their appearances are easily stained. Therefore, related industries have developed continuous researches as a plan for improving contamination, in order to combine self-cleaning property applied to building materials with steel sheets for air conditioners.
Self-cleaning steel sheets are steel sheets for providing them with a function to clean pollution sources on a coated film by rainwater, and the like, for themselves, and may be classified broadly into hydrophilic self-cleaning steel sheets, photocatalytically hydrophilic self-cleaning steel sheets, hydrophobic self-cleaning steel sheets, and the like, depending on self-cleaning principle.
The hydrophilic self-cleaning steel sheets are steel sheets which clean pollution sources by lowering a water contact angle in a surface of a coating layer (contact angle: 40 degrees or less, surface tension: 30 mN/m or more) to scatter rainwater, and the like, evenly over all the surface of the coated film, and provide a surface of a coating layer by 1˜2 μm with hydrophilicity, using hydrophilic organic polymers. Said hydrophilic organic polymers may include polyalkylsilicate, perhydropolysilazane, and the like.
The photocatalytically hydrophilic self-cleaning steel sheets are steel sheets which degrade organics using TiO2, and the like, having a photo activation effect and then provide self-cleaning property thereto using hydrophilicity of a coating layer itself.
The hydrophobic self-cleaning steel sheets are steel sheets which clean pollution sources on a surface of a coated film by increasing water contact angle in a surface of a coating layer (contact angle: 120 degrees or more, surface tension: 15 mN/m or less) to repel and roll down rainwater. Depending on a configuration method, there are a hydrophobic coating (fluorine-based coating, and the like), a nanostructure coating, a plasma surface modification method, and the like. Said hydrophobic coating is a coating that materials such as fluorine-based materials having high hydrophobicity are focused on a surface of a coating layer to provide hydrophobicity thereto, said nanostructure coating is a coating utilizing a leaf structure in a lotus flower, which provides projections in a micro size with a ciliated structure in a nano size to represent superhydrophobicity, and said plasma surface modification method is a method which provides a surface with hydrophobicity through a plasma treatment.
The self-cleaning steel sheet coated with a hydrophilic organic polymer is a steel sheet that an organic resin in a hydrophilic thick film (a thickness of about 20 μm) is focused on a surface of the coated film to wash and clean contaminants with rainwater, and the like. There is an advantage that it is inexpensive and easily ensures anticorrosion and antistatic effect, while there is an disadvantage that a film thickness of at least 5 μm should be required to ensure other physical properties.
In addition, the photocatalytically hydrophilic self-cleaning steel sheet using TiO2 is a steel sheet that contaminants are degraded and cleaned with surface oxidation action of a photocatalyst by ultraviolet, which is capable of being formed in a thin film (<1 μm) and useful in removing NOx, SOx, and the like, but there is an disadvantage of representing self-cleaning property via photoreaction only under irradiation of ultraviolet.
Furthermore, it is noted that the photocatalytically hydrophilic self-cleaning steel sheet represents superhydrophilicity using porous silica-metal nanocomposites. Said self-cleaning steel sheet representing superhydrophilicity may exhibit superhydrophilicity without irradiating it with ultraviolet. Thus, there is an advantage that it may be applied to interior application fields as well as exterior application fields, while there is a disadvantage that development of applicable technology in coating steel sheets is required.
The fluorine-based hydrophobic self-cleaning steel sheet of hydrophobic self-cleaning steel sheets is a steel sheet that a fluorine compound on a surface gives hydrophobicity thereto to remove contaminants with water. There is an advantage that deposit of contaminants is low, while there is a disadvantage that a fluorine-based compound is difficult to be focused on a surface and flushing capacity is lowered under pollution environment in oil stains. In addition, the hydrophobic self-cleaning steel sheet with a nano projection structure is a steel sheet that hydrophobic nano projections are formed to remove contaminants with water. There is an advantage that deposit of contaminants is low and flushing capacity is good, but there is a disadvantage that after processing, it is difficult to maintain the nano projection structure, that is, durability is poor.
Moreover, in addition to each disadvantage as described above, there is a problem that they have limits on applying to application fields requiring good anticorrosion and processability, and the like, since the conventional self-cleaning steel sheets as described above consider only the self-cleaning property.
Accordingly, the present invention is intended to provide a self-cleaning article having excellent anticorrosion and processability as well as excellent self-cleaning property and a process for preparing the same.